Slew rate is an important characteristic of the design and operation of integrated circuit devices, especially output buffers in high speed serial transceivers. Generally, the slew rate is the rate at which the output from an electronic circuit or device can be driven from one limit to another limit over the dynamic range of the electronic circuit or device. For the output buffer of a serial transceiver, the slew rate is defined as (V1−V0)/TR for a rising output transition, and as (V1−V0)/TF for a falling output transition, where V0 and V1 are the output levels associated with a logic 0 and logic 1, respectively, and TR and TF are the output rise and fall times, respectively. If the slew rate of a high speed serial output buffer is too low, then the maximum output rate of the buffer will be limited. However, if the slew rate is too high, then the frequency spectrum of the output waveform will have significant high frequency components, causing the buffer to act as a source of electromagnetic interference and crosstalk noise. A number of standards for high-speed serial transceivers impose both upper and lower limits on the rise and fall times, and thus on the slew rate, of the serial output waveform.
A number of techniques have thus been proposed or suggested for controlling the rise and fall times of a signal. The rise and fall times of the output buffers is often a function of the power supply voltage, as well as process and temperature variations within the integrated circuit device. Conventional rise and fall time control techniques tend to be sensitive to process, supply voltage and temperature variations, and cannot be adjusted very precisely.
Typically, rise and fall time control in an output buffer is implemented by placing a number of analog delay cells connected in series in the data path. The output driver is split into a set of smaller driver cells with outputs connected in parallel, each of which is driven by the output of one of the delay cells in the data path. The delay elements generate a corresponding number of time shifted versions of the data signal, which are then aggregated to provide a stepped version of the data signal. Generally, the total delay through the delay elements determines the rise and fall times, while the number of delay elements determines the smoothness.
The inclusion of delay elements in the data path, however, causes data dependent jitter. Generally, when random data is passed through bandwidth-limited analog delay elements, there is a tendency to stretch wide pulses (such as associated with a string of binary ones) and to compress narrow pulses (such as a binary one between strings of binary zeros).
A need therefore exists for improved rise and fall time control techniques that do not rely on delay elements in the data path. A further need exists for rise and fall time control techniques that can adjust the rise and fall times of the output buffers over a relatively large range so that a slew rate can be selected based on the application data rate. A further need exists for rise and fall time control techniques that can control the rise and fall times of an output buffer so that they are substantially independent of supply voltage, operating temperature, and processing.